The present invention relates to a fiber composite semifinished product, and to a method of producing fiber composite semifinished products by means of a circular braiding technique.
Different methods of producing tube-shaped braidings (German Patent Document DE-A-42 34 979) or three-dimensional braidings (U.S. Pat. No. 5,630,349) are known. Because of the fact that braiding fibers are wound onto a braiding core, such circular braidings naturally have a so-called linen or body texture. This results in a waviness of the braiding fibers, such that the positive features of the fibers, specifically a high tensile and compressive stiffness or a high tensile and compressive strength, cannot be optimally utilized in a fiber composite material produced by a conventional braiding technique.
In addition, the known methods have the disadvantage that the braiding fibers are damaged or weakened by the shearing forces applied to them during the braiding or as a result of the friction at corresponding crossover points. These effects can be reduced, for example, by a braiding machine having two braiding rings (German Patent Document DE-C-101 15 935) carrying out periodic stroke movements. However, this arrangement still has the problem of the waviness of the fibers.
European Patent Document EP 0 628 401 A1 discloses a method for manufacturing a product (particularly a sports device) made of a fiber-reinforced thermoplastic resin, as well as a corresponding component, in which matrix-forming fibers and reinforcing fibers are woven or braided together.
International Patent Document WO 92/15740 A1 discloses an asymmetrical braiding for improving fiber-reinforced products.
One object of the present invention, therefore, is to provide a method of producing braided fiber composite semifinished products which reduces both the fiber damage and the waviness of the fibers, with improved characteristics of the material.
This and other objects and advantages are achieved by the production technique and the fiber composite semi-finished product according to the invention, which are based on the circular braiding technique, in which a braiding core is braided with braiding threads that are unwound by means of bobbins circling concentrically about the braiding core in different directions. According to the invention, the bobbins of one circling direction are fitted with reinforcing threads and the bobbins of the opposite circling direction are at least partially fitted with supporting threads, which are formed at least in part by thermoplastic threads.
By the braiding-in of thermoplastic threads (which, as known, consist of plastic materials, such as polyamides, polystyrenes, polyethylenes, polyesters, etc. that melt when heated over the softening point, and can be hot-formed, solidify again after cooling, and have good sliding characteristics) first, the friction of the mutually crossing braiding threads is reduced because the reinforcing threads slide off with reduced friction on the thermoplastic threads. This results in a clear reduction of the fiber damage, and therefore in an improvement of the material characteristics of the braiding.
Expediently, the supporting threads, formed at least partially of thermoplastic threads, hold the reinforcing threads deposited on the fiber core in position, so that the flexibility of the braiding process with respect to the braiding core geometry is simultaneously ensured. In this case, the elastic thermoplastic threads are placed so snugly between the reinforcing threads that the latter come to be situated in parallel virtually without any space in-between, and are therefore deposited almost without any waves. As a result of the accompanying reduced fiber waviness, the positive features of the reinforcing threads can be optimized, so that the material features of fiber composite semifinished products produced according to the invention are considerably improved.
Expediently, the braiding core is braided several times successively, with individual unidirectional reinforcing fiber layers being in each case deposited on the braiding core. The term “unidirectional” means that plane, not wavy individual layers are involved. This, in turn, has the advantage that the computability of the fiber construction of braidings produced according to the invention is improved because the mathematical models for unidirectional layouts can be applied. In addition, the thickness of such individual layers is reduced by approximately half in comparison to a braiding produced by means of a conventional braiding technique; that is, all bobbins are occupied by reinforcing threads.
It is another advantage that, during a layer-type braiding of the braiding core, before the depositing of another individual layer, the previously deposited individual layer can be fixed by melting the thermoplastic threads. As a result, sliding-out-of-place or displacement is prevented simply and effectively. The melting can be achieved, for example, by local heating or by the application of a vacuum hose with subsequent heating. In the latter case, the deposited individual layer is correspondingly consolidated, which further reduces the waviness of the braiding.
An asymmetrical bobbin occupation is expediently conceivable, during which the number of bobbins circling in one direction is unequal to the number of bobbins circling in the opposite direction, which ensures a great degree of variation. If, for example, reinforcing threads are placed on three quarters of the bobbins circling in one direction and thermoplastic threads are placed on one quarter of the bobbins circling in the opposite direction, one-and-a-half times the number of reinforcing threads can be processed in an individual layer. As a result, a depositing width is increased by 50%, and the braidable core circumference increases to the same extent. This has the advantage that correspondingly smaller and therefore less expensive machines can be used.
Typically, the reinforcing threads consist of carbon, glass, aramid and/or Kevlar fibers, which are characterized by high tensile and compressive stiffness as well as high tensile and compressive strength.
It is particularly advantageous that the supporting threads completely or at least partially dissolve at temperatures at which the braiding is normally infiltrated. Depending on the application, the supporting threads are made completely or at least partially of Grilon® threads or other thermoplastic threads with melting temperatures in the range of the infiltration temperature. In addition, the supporting threads may also be made of materials which are only partially liquescent.
However, as an alternative, thermoplastic threads can also be used which have a melting point above the typical infiltration temperature (such as polyester fibers). Such supporting threads do not dissolve in the matrix system of the infiltrated braiding, so that targeted feeding of supporting threads becomes possible, which may be advantageous for some applications.
The braided fiber composite semifinished products according to the invention, are made of a plurality of unidirectional individual layers, deposited layer by layer, each individual layer having braided-in supporting threads consisting at least partially of thermoplastic threads. By an appropriate selection of the supporting threads, special demands can be met advantageously, in a simple manner, so that the supporting threads in the infiltrated braiding are either completely or partially dissolved or are not dissolved at all.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.